We have a near twin car, 21mpg highway

Ours is a 64-1/2, stock 289, 4100 1.12 Autolite, c-4, 3.00 gear. On the highway at 60mph, trips of 50 miles or more it will get 20-21mpg. In the city, it can get as low as 12mpg (trips of a couple miles in cold weather, maybe 16mpg tops. So it really depends on the driving.

But on a long trip at 60mph yes you should be close to 20mpg if its tuned right. At 70mph it would drop to maybe 18mpg highway.

Make sure it is jetted until the exhaust pipes look right (not black but light gray after a long trip) and the plugs stay clean. And, total advance (initial plus mechanical plus vacuum) should be 40 to 50 degrees at cruise rpm, any more or less than that and it will not be happy.